On Monday, May 27, 2019 at 11:22:19 PM UTC+1, Nicholas Randall wrote:
On Sunday, 26 May 2019 20:38:33 UTC+1, Nick Gardner wrote:
Today and yesterday have seen quite high UV levels after what has been a
spring season of rather normal UV intensity.

The lack of tropical maritime air masses has probably been responsible.

The UV here today reached 8.6 despite quite cloudy and hazy conditions
with the sun always being partly obscured by haze or cloud. Yesterday's
hazy conditions saw the UV reach 8.0.

I suspect that now the CF has moved through this evening and we're in a
different air mass, the UV maximum tomorrow will be down to around 6.0
to 7.0.

Given the forecast for later this week, we may see an extremely high UV
level of +9.0 on Wednesday. Slip, slop, slap...oooerrrrr.

--
Nick Gardner
Otter Valley, Devon
20 m amsl
http://www.ottervalleyweather.me.uk

UV levels are not related to temperature so they would not be lower because of a colder air mass and they would not be higher because of a warmer air mass. They relate to the position of the sun in the sky. At this time of year, it is at its highest point at 1 pm and this is when UV levels can be high.

Nicholas
Meir Heath, Stoke-On-Trent 250 metres above sea level.

UV levels are not related to temperature, agreed. The height of the sun in the sky is by no means the only factor, air mass is important. Warm sector ridges are associated with very high UV levels. The reason for this is open to discussion, but it's indisputable. To people like Nick, who has a sunshine recorder it's blindingly obvious. For me, who keeps an eye on Camborne UV it's fairly dramatic.

Saturday it was sunny at Camborne around 13:00, visibility was M to G with patchy low cloud, hazy conditions, sea fog patches nearby and fairly weak shadows. UV hit 9. Yesterday also sunny around the same time, VG visibility, deeper colours, lower humidity, sharper shadows, UV 7. Under a hot, dirty continental air mass, it would be even lower.

I've had discussions with people like Martin Rowley in the past, and researched it. The generally accepted explanation seems to be that it's associated with upper atmospheric conditions under warm sector ridges. However, I'm of the view that the low level near 100% humidity also has a role to play.

I think anyone who spends any time on the water when there are sea mist patches on a warm summer's day, knows how quickly you get burnt. It's an issue for me with a bald head. People like Norman L have also commented on it on USW.

If in doubt, I suggest you check Camborne UV levels from late May to the end of July over 2 or 3 years, then compare the very high readings with the synoptic chart. I suspect NIck G has records you could also compare. Also records from Scilly http://www.scillyman.co.uk/WeatherStation.html (if he kept them). The association is striking.

Graham
Penzance